Why Are Some Floral Nectars Scented?

Despite recent interest in the non-sugar components of floral nectar, nearly nothing is known about the ecological importance and phylogenetic distribution of scented nectar. If present, the scent of nectar would provide an honest signal to nectar-feeding animals. Nectar odors may directly impact plant reproductive fitness, through pollinator attraction or deterrence of nectar robbers and florivores. In addition, nectar odors may indirectly impact plant fitness through antimicrobial activity, pleiotropic interactions with plant defense, and communication with predators and parasitoids. The literature provides only circumstantial evidence for scented nectar, through the study of bee honey odors. Here I confirm the presence of scent in the nectar of four out of seven angiosperm species sampled with solid-phase micro-extraction and gas chromatography–mass spectrometry. In Abelia x grandiflora and Hedychium coronarium, nectar odors are a hydrophilic subset of the compounds emitted by surrounding floral tissues, suggesting passive absorption by the nectar standing crop. Sucrose solution applied to the petals of a nectarless flower, Magnolia grandiflora, absorbed a hydrophilic subset of scent compounds after one hour, lending support to this hypothesis. Nectar from Oenothera primiveris and Agave palmeri contained unique scent compounds compared to the floral tissues. The presence of fermentation volatiles in A. palmeri nectar suggests a dynamic role for yeast in its floral biology. These data highlight the need for systematic studies on the distribution and mechanistic importance of scented floral nectar to plant–animal interactions.